1. Field of the Invention
This invention relates to a method and apparatus for increasing the effective bandwidth of isochronous, ring networks, and more particularly to a method and apparatus for increasing the effective bandwidth of networks conforming to FDDI-II standards.
2. Description of the Prior Art
The Fiber Distributed Data Interface (FDDI-II) standard prescribes a high-performance, fiber optic, isochronous, ring Local Area Network (LAN) running at 100 megabits per second (Mbps). Such networks provide a high bandwidth, general purpose interconnection among, for example, processors. However, even 100 Mbps is not sufficient bandwidth for many high data rate applications that exist today. In some client/server applications, such as those which manage large video disk arrays, the isochronous bandwidth requirement of the server for all the clients on the network is in excess of 100 Mbps. At the same time, it is highly desirable that such client/server networks conform to existing FDDI-II standards, using commercially available FDDI-II hardware, cabling, cycle structure, and protocols.
FDDI-II is a high-performance, fiber optic, token ring LAN running at 100 MBS. FDDI-II is a successor to FDDI with a modification to handle synchronous circuit-switched pulse code modulated (PCM) data for voice or ISDN traffic, in addition to ordinary data. In addition to regular (asynchronous) frames for ordinary data, special frames are permitted for isochronous circuit switched data. These frames are generated every 125 microseconds by a master station. The FDDI-II cycle structure is defined in Section 5 of the ANSI FDDI Hybrid Ring Control standard (X3T9/89-43 Rev 6.3) and incorporated herein by reference. Each new cycle or frame consists of control and data bytes or octets. The master station adjusts the latency of the FDDI-II ring so that there are always an integral number of these frames or cycles on the ring. Each frame is partitioned into four parts: a preamble, a cycle header, a dedicated packet group, and sixteen, wideband channels (WBCs). Each wideband channel can be dynamically allocated to either isochronous or packet data use. For isochronous use, each wideband channel provides 6.144 Mbps of bandwidth, and consists of 96 octets interleaved across the cycle. Once a station has acquired one or more time slots in a frame, those slots are reserved for it until they are explicitly released. The allocation of bandwidth to isochronous users is defined in Section 9.3 of the ANSI FDDI Station Management-2 Isochronous Services (SMT-2-IS) standard (X3T9/93-122 Rev 3.0).
The FDDI-II ring is a collection of ring interfaces connected by point-to-point lines. Each bit arriving at an interface is copied into a buffer and then copied out onto the ring again. This copying step introduces a 1-bit delay at each interface. A token (i.e. a special bit pattern) circulates around the ring whenever all stations are idle. When a station wants to transmit a frame of packet data, it is required to first seize the token and remove it from the ring before transmitting. Bits that have been propagated around the ring come back to the transmitting interface and are removed from the ring by the sender. In FDDI-II, a transmitting-interface may put a new token on the ring as soon as it has finished transmitting its frames.
Isochronous WBCs can be subdivided into individual transmission channels. A transmission channel consists of a contiguous sequence of octets within a WBC. Each data octet of a transmission channel provides 64 kbps of isochronous bandwidth. A logical channel is a unit of allocation that may consist of one or more transmission channels. WBCs are subdivided into separate logical channels to permit simultaneous, independent isochronous dialogues.
An FDDI-II management agent, such as a call control, must receive an allocation of isochronous bandwidth before completing a connection between two or more isochronous users. It is the responsibility of a Channel Allocator (CA) to create logical channels and assign isochronous bandwidth to those channels for use by call control.
Existing data base management routines are useful to a channel allocator in optimizing the way logical channels are created from a pool of bandwidth. One way that a channel allocator can manage bandwidth allocation for the different physical ring domains is defined in U.S. Pat. No. 5,422,883 which is incorporated herein by reference. The use of such routines is outside the scope of this patent. The operation of a channel allocator as defined in the FDDI-II standards will require modifications to account for the different physical ring domains that are described below.
U.S. Pat. No. 5,517,498 discloses a method of extending the effective bandwidth of a ring network connecting a plurality of stations connected one to the next. Here, some or all of the stations on the network comprise a group of contiguous stations that only communicate with one another. Outside the group, the bandwidth assigned to the group can be assigned to another such group.